The present invention relates to microwave apparatus. More specifically, the present invention relates to apparatus for the transmission and reception of more than one microwave beam at a time. An example of such apparatus is a single antenna which can transmit and receive two or more microwave beams in different directions.
The cost and practical convenience of multiple beam apparatus are important determinants of its utility. Accordingly, the utility can be enhanced by application of inventive concepts which reduce cost by providing a more compact structure composed of smaller parts and which provide practical convenience by allowing for many convenient locations for the parts.
The radiation gathered by a multiple beam antenna in its receive mode must be efficiently conducted through the structure. Undesirable dispersal, blockage, or loss of the radiation necessitates a larger structure and more widely spaced beams as compared with an antenna not exhibiting such undesirable characteristics. Likewise, transmitted beams must not be allowed to spill over or disperse in unintended directions.
A multiple beam antenna having low blockage and spillover effects is disclosed in my U.S. Pat. No. 3,914,768 "Multiple-Beam Cassegrainian Antenna," issued Oct. 21, 1975, and assigned to the assignee hereof. Feed horns transmit beams which are centered on a paraboloidal main reflector after reflection by a hyperboloidal subreflector enlarged and located to eliminate spillover and blockage. The centering of beams permits a smaller main reflector than is required in an antenna having the beams set with centers apart.
The feed horns can efficiently receive beam energy from the entire main reflector surface due to the centering of the radiation pattern of each horn on the main reflector. The beams of received radiation are focused into their respective horns. The focus must be properly located to achieve compactness and convenience in mounting, adjustment, and location of the feed system.
The minimum size of the main reflector has heretofore been limited by the physical size of the feed horns into which the beams are focused. Each feed horn has an aperture for receiving one of the beams. The beams are unrecoverable separately when the horn apertures required for good coupling are too large because the beams cannot then be centered in the horns with the horns side by side. It is necessary either to use a larger main reflector to feed the beams to the horns, or to use larger beam spacing angles. Either way, the horns must be located near one another in an arrangement which can prove to be inconvenient in practice.
Accordingly, an object of the present invention is to eliminate feed horn size as a constraining factor in multiple beam antenna design.
Another object of the present invention is to increase design freedom in the location of feed horns in a multiple beam antenna.
Another object of the present invention is to minimize the size of the main reflector in a multiple beam antenna having a given beam spacing.
Another object of the present invention is to reduce the minimum spacing of multiple beams which may be received and transmitted by a multiple beam antenna having a given main reflector size.